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Influence of Earthing Resistance on the Performance

of Distribution Line Lightning Arrester N.A. Abd. Rahman

#1, N. Abdullah

*2, M.F. Ariffin

#3

# TNB Research Sdn. Bhd.

No. 1, Lorong Air Itam

Kawasan Institusi Penyelidikan, Kajang

43000 Selangor, [email protected]

[email protected]

*Tenaga Nasional Berhad,

11th

Floor, Wisma TNB, No. 19

Jalan Timur, Petaling Jaya

46200 Selangor, [email protected]

Abstract —

The line lightning performance of the 33kV distribution line

Tanjung Batu - Rompin gives very bad performance during

lightning. The lines have bad performance although they were

equipped with the line lightning arrester to limit the overvoltages

on the phase conductors when lightning strikes the line. There

are several cases where the surge arresters were broken and

exploded. In this study, the effect of earthing resistance on the

surge arrester was studied and simulated using ATP-EMTP to

determine the performance of the line lightning arrester in

limiting the lightning voltages on the phase conductor.

I. I NTRODUCTION

Lightning is the major cause of the 33kV Tanjung Batu –

Rompin (TJBU – RMPN) breakdown occurrences. The line is

approximately 55km long running through the rural area of

Pahang, located at East Coast of Peninsular Malaysia. From

the record, the total number of breakdown from the year 2002

to 2006 is 64 breakdowns. The line breakdown due tolightning is 30 which is about 47% from the total number of

line breakdown.

Based on Lightning Detection System Lab (LDS), TNB

Research Sdn. Bhd. (TNBR) records from January 2004 toSeptember 2008 indicate that the average ground stroke

densities of the area on which the line route range from 13 –

15 strokes/km2/year.

Fig. 1 Ground strokes density for TJBU-RMPN line traverse

II. BRIEF DESCRIPTION OF 33KV TANJUNG BATU – R OMPIN

DISTRIBUTION LINE

The 33kV Tanjung Batu – Rompin line length is

approximately 55km. The line has 413 poles which theaverage span length of 150m. The measured soil resistivity

along the line is varies between 10 to 600 Ω.m. A total of 22MOV line surge arresters have been installed on the line. The

phase conductors are single conductor of 150mm2 Silmalec.

This line has no shield wires installed, thus no dedicated

earthing system installed at every pole. The Basic Insulation

Level (BIL) of the line is 170kV.

2010 Asia-Pacific International Symposium on Electromagnetic Compatibility, April 12 - 16, 2010, Beijing, China

978-1-4244-5623-9/10/$26.00 ©2010 IEEE 1538

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Fig. 2 Surge Arresters Installed on the Pole

III. SIMULATION DATA

A total of seven poles which are located approximately in

the middle of the 33kV Tanjung Batu-Rompin line are

selected and modelled in the ATP-EMTP. The phase

conductors at both ends are terminated by a 450 ohm

resistance which represents the surge impedance of theconductors for the remaining part of the circuit.

In this simulation, we study the case in which lightning

strikes Phase A (outer phase) in the mid-span between Pole

No.4 and No.5, where lightning surge arresters are installed atthe phases of Pole No. 4. It is expected that Phases A and C

are more susceptible to lightning strikes, as compared to Phase

B (middle phase). For unshielded horizontal circuits, both

outer phase wires will be more exposed to direct lightning

strikes, which would provide the middle phase with some

degree of shielding from direct strikes. Protecting the outer phases of a flat horizontal circuit with surge arresters could be

an effective means for improving line overall lightning performance.

Fig. 3 Modelling of Seven Distribution Poles in ATP EMTP

The lightning surge modelled in this simulation is

characterised by 8/20µs, Heidler current waveform, with a

peak magnitude 21kA which is the mean value of lightning

current at the traverse of Tanjung Batu – Rompin line for the

year 2007.

The line surge arrester currently installed is a gapless metal

oxide varistor. The rated voltage is 29kV and the MCOV

(Maximum Continuous Operating Voltage) is 36kV.

0

5

10

15

20

25

30

35

40

45

104 112 120 132 156 202

Current (kA)

V o l t a g e ( k V )

Fig. 4 Voltage – Current Characteristics of MOV Surge Arrester

IV. R ESULTS AND DISCUSSIONS

Figure 2 shows the voltage on Phase A when lightning

strikes the conductor. The maximum overvoltages on the line

is about 4.8 MV which exceeds the line BIL of 170kV.

Fig. 5 Overvoltages on Phase A when 21kA lightning strikes the phaseconductor

For the purpose of determining the importance of pole

earthing, 10kA and 15kA lightning surge currents are used. Table 1 below shows the overvoltages on Phase A conductor

when a 10kA lightning strikes the conductor. The maximum

overvoltages for lightning surge current of 10kA and 15kA are

about 2.29MV and 3.43MV respectively, which exceed the

line BIL of 170kV. Surge arresters are recommended to be

installed to prevent the current surges from flowing to thenetwork by diverting them harmlessly to the ground. They

would also limit the overvoltages to the lowest possible level.

TABLE I

LINE OVERVOLTAGES WHEN 10K A AND 15K A LIGHTNING SURGE CURRENTS

STRIKE THE PHASE CONDUCTOR

Overvoltages (MV)

Lightning Surge

= 10kA

Lightning Surge

= 15kA

WithoutArrester

2.29 3.43

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However, to install the surge arresters, earthing resistance

must be maintained at low or recommended value to ensure anoptimized lightning surge operation. Table 2 below shows the

overvoltages on Phase A conductor when 10kA and 15kA

currents of lightning strike the conductor with different values

of pole earthing.

TABLE IILINE OVERVOLTAGES WHEN 10K A AND 15K A LIGHTNING SURGE CURRENTS

STRIKE PHASE CONDUCTOR

Pole Earth

Resistance ( )

Overvoltages (kV)

Lightning Surge =

10kA

Lightning Surge =

15kA

0 131 144

1 140 158

5 176 213

10 219 277

15 258 336

20 290 391

30 360 487

40 416 571

50 465 644100 661 935

200 875 1251

Table 2 shows the overvoltages on the phase conductor

when surge arresters are grounded with different earthing

resistance values. The overvoltage on the line exceeds the line

BIL even when the earthing resistance is only 5 Ω. Therefore,it is crucial to have low earthing resistance since it will affect

the surge arrester operation effectiveness in limiting the line

overvoltages.

Line Lightning Overvoltages

0

200

400

600

800

1000

1200

1400

0 1 5 10 15 20 30 40 50 100 200

Earth Resistance (ohm)

O v e r v o l t a g e s ( k V )

10kA

15 kA

Fig. 6 Earth Resistances vs Line Overvoltages

From the Figure 3, it shows that at low earth resistance

value, the overvoltages on the phase conductor at different

value of lightning current shows not much different. The

performance of the line surge arrester in limiting the

overvoltages will decrease to about 5 % of every 5 ohm

increase.

The overvoltages on the line were analyzed with different

value of lightning magnitude. Table 3 shows the overvoltages

at the phase conductor at various magnitude of lightning

magnitude when the line surge arrester has good grounding

system which near to 0Ω and 20Ω.

TABLE III

LINE OVERVOLTAGES WHEN ARRESTERS GROUNDED WITH DIRECT GROUND

AND 20 Ω EARTHING RESISTANCE

Lightning

current (kA)

Overvoltages

(kV)Direct Ground (0 ) 20

10 130 295

11 133 314

12 136 333

13 139 352

14 142 372

15 144 391

16 146 410

Line Lightning Overvoltages

0

100

200

300

400

500

600

10 11 12 13 14 15 16

Lightning Current

O v e r v o l t a g e s ( k V )

20 ohm

~ 0 ohm

Fig. 7 Lightning Current vs Line Overvoltages

The higher the magnitude of the lightning current that

strikes the phase conductor, the higher lightning overvoltageson the line would be occurred on the line. Also, the higher theearth resistance of the arrester, the higher the line lightning

overvoltages would be.

However, it can be seen in Figure 3 that, when line surge

arrester has very good earthing system which the earth

resistance value is 0Ω, the increment of the line overvoltages

value at different lightning current magnitude are negligible.

The performance of line surge arrester is the best when it isgrounded with a very low earth resistance.

V. CONCLUSIONS

When the surge arrester is installed on the line, it will limit

the lightning overvoltage magnitude to about 95%. However,

the effectiveness of the surge arrester will decrease when the

resistance of the surge arrester to the earth increases. With an

increase of every 5 ohm, the performance of surge arrester to

limit the lightning overvoltages decreases by about 5%. These

results show that the effectiveness of line surge arresters is

affected by the earth resistance. To ensure the line surge

arrester is operated at the best performance, regardless the

lightning magnitude that will hit the line, earth resistance must

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be kept very low since the performance of the line surge

arrester is dependable on its earthing system value.

ACKNOWLEDGMENT

An acknowledgement is made to all relevant people who

have contributed towards the success of this study. The author

also would like to thank the Management of TNBR and

TNBD for providing the support for this study. Thanks alsoTNBR research team members who have worked very hard to

complete this project.

R EFERENCES

[1] IEEE Std. 1243 – 1997, IEEE Guide for Improving the LightningPerformance of Transmission Lines

[2] A.P. Sakis Meliopoulus, John Kennedy, “Statistical Lightning

Performance Evaluation of Overhead Distribution Lines”, IEEE, 2000[3] F.P. Dwalibi, W. Ruan, S. Fortin And J. Ma, “Computation of Power

Line Structure Surge Impedances Using the Electromagnetic Field

Method”, IEEE 2001[4] F.P. Dwalibi & P. Hotte, “Distribution Line Safety Grounding:

Parametric Analysis”, IEEE1998[5] F.P. Dwalibi, W. Ruan, S. Fortin, J. Ma & W.K. Daily, “Computation

of Power Line Structure Surge Impedances Using the Electromagnetic

Field Method”, IEEE 2001

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